EP1881083B1 - Workpiece made of a high-strength steel alloy and its use - Google Patents

Abstract

Workpiece made from a high strength steel alloy contains (in wt.%) 0.11-0.18 carbon, 0.10-0.30 silicon, 1.60-2.20 manganese, less than 0.0015 phosphorus, less than 0.010 sulfur, 1.00-2.00 chromium, less than 0.020 nitrogen, 0.020-0.060 niobium, 0.001-0.004 boron, 0.001-0.050 titanium and a balance of iron. A semi-finished product in the form of sheet metal or tube with a ductile ferritic-perlitic structure is heat treated by austenizing, cooling in air and then optionally tempered. The workpiece has a tensile strength Rm of more than 1.200 MPa at a fraction elongation A5 of more than 12 %.

Description

The invention relates to a workpiece made of a high-strength steel alloy and its manufacture and use.

Air hardening steel alloys have the advantage that they already have a high strength in the normalized state. They are only slightly influenced by the strength of joining processes such as welding and soldering and at the same time have a high tempering resistance in the hot dip galvanizing, z. B. in the high-temperature galvanizing.

There are already a number of applications have been proposed in which air-hardening steel materials can be used with good toughness. In the DE 102 21 487 84 the use of an air-hardening steel material for lightweight steel construction is proposed. The use for pipes of motor vehicles has also been described ( EP 1565 589 81 ). Air hardening steel materials are also suitable for the production of thick-walled pipe components ( EP 1 565 588 B1 ). Hardening by quenching an air-hardening alloy in the tool is described in US Pat DE 102 61 21 0 A1 described. Another alloy concept is the subject of DE 10 2004 053 620 A1 , which relates to a high-strength air-hardening steel with good forming properties for use in lightweight vehicle construction.

By the DE 24 52 486 C2 It is the state of the art to heat a steel sheet with a small material thickness to a temperature above Ac3, then press it into the final shape between two indirectly cooled tools with substantial change in shape and to undergo rapid cooling, while remaining in the press, that a martensitic and / or bainitic fine-grained structure is achieved (tool hardening). This thermoforming process combined with tool hardening gives a product with good dimensional stability and high strength.

Classic air-hardening steel materials are based on the CrMoV concept with Mo contents between 0.4% and 0.6% and V contents between 0.1% and 0.2%. Due to the sharp increase in raw material costs, classic CrMoV steels are in the range of mass applications, such as e.g. in the automotive industry, generally no longer economically viable.

An alternative alloying concept is the MnTiB concept, which is known from water-hardening steels. By adding molybdenum and tungsten to the MnTiB concept, as in the DE 10 2004 053 620 A1 proposed, air-hardening properties are achieved at low C-contents at high elongation at break. However, in these alloy concepts, the raw material costs for the alloying elements are very high. In addition, it can be stated that although air-hardening materials with C contents of between 0.09% and 0.12% achieve considerable strengths of up to 1200 MPa and a comparatively high elongation at break or toughness due to the low C contents, they undergo rapid cooling. However, in continuous casting tend to above-average failure rates due to poor surfaces. The reason is to be seen in the peritectic solidification with the result that a reworking of the material is necessary.

On this basis, the invention is based on the object to show a workpiece using a high-strength, air-hardening steel alloy with a cost-effective alloy concept and its production, which is particularly suitable for use in motor vehicle construction.

This object is achieved by a workpiece according to the features of patent claim 1. In the steel alloy according to the invention, the C content is raised to 0.11% to 0.18% in order to achieve higher strengths and moreover to avoid the disadvantages of peritectic solidification. In this way, better surfaces can be achieved and rework avoided. Also, the failure rates are reduced in continuous casting.

The setting of the hardenability in the alloy type is carried out primarily by adding manganese and chromium in cooperation with titanium and niobium. Essential for achieving a high elongation at break with high tensile strengths is the addition of niobium and the adjustment of a controlled N content together with titanium, niobium and boron and a suitable thermomechanical treatment of the material. In this procedure can be dispensed with the costly alloying elements molybdenum and vanadium.

For example, with a target strength of 1,300 MPa, the microalloying concept allows the A5 elongation at break to be increased from 10% to 16% through targeted thermomechanical treatment during hot strip production and also during the production of seamless pipes. The thermomechanical treatment provides for control of rolling temperatures, picking and cooling control after rolling. This allows a bainitic structure with bainite contents greater than 80% with particle sizes below 10 microns and especially below 5 microns can be adjusted so that there are high elongation at break values. However, the high strength can be adjusted not only by the thermomechanical treatment, but also by an additional heat treatment, in particular by hardening and tempering.

In the production of the workpiece, a semifinished product of the alloy of sheet metal or tube according to the invention can first be used in the soft, ferritic-pearlitic state. This condition can be achieved by an annealing treatment or by setting suitable reel temperatures during belt production. The soft ferritic-pearlitic state is characterized by a yield strength Rp 0.2 of 500 MPa and a high elongation at break A5 of more than 25% and allows a very good cold working of the semi-finished products. In a subsequent heat treatment by austenitizing above Ac3 and cooling in air high strengths are achieved Depending on the workpiece geometry and the resulting cooling conditions strengths Rm in the range of 1100 MPa to 1400 MPa at yield strengths in the range 850 MPa to 1050 MPa and strains A5 in the range 10% - 16% achieved. By a subsequent tempering treatment each desired strength / expansion ratios can be adjusted. For example, a subsequent tempering treatment at 600 ° C reduces the strength to 880 MPa at an elongation A5 of 16%.

The increased process reliability during thermoforming or hardening in the forming tool is a significant advantage in the production of the workpieces from the steel alloy according to the invention. Here, the problem may arise that arise between the cold tool and the abhorred structural component geometry-dependent air gaps, which inevitably adversely affect the homogeneity of the desired high cooling rate in the forming tool.

In contrast, an inventive workpiece made of the air-hardening steel material can be produced by thermoforming with increased process reliability getting produced. Workpieces made of conventional water-quenched materials sometimes show local tensile strengths Rm of less than 850 MPa in areas which, due to errors in the tool geometry, led to uneven cooling conditions. In contrast, the material according to the invention exhibits a homogeneous tensile strength of more than 1,300 MPa in the tool-tempered state, even in the case of significant irregularities in the process.

In addition, workpieces according to the invention have advantages in terms of weldability and in terms of tempering behavior. Usual steels for tooling, e.g. a 25MnB5 steel show disadvantageously in the region of the weld hard toes and in the heat affected zone a hardness drop, so that the component properties are significantly deteriorated in welded areas, So is e.g. For workpieces from a 25MnB5, a drop in hardness to well below 300 HV can be observed. Welding tests on workpieces according to the invention have shown that in the area of the welding zone and the heat-affected zone of the air-hardened or tool-tempered components, no appreciable hardening and no appreciable hardness drop are to be observed.

Thermoformed or air hardened workpieces of the present invention may also provide corrosion protection benefits, although it is generally difficult to achieve the highest corrosion protection requirements for thermoformed components. Highest corrosion protection requirements can be achieved with zinc coatings. A conventional electrochemical galvanizing of high-strength thermoformed workpieces is not possible, since there is an embrittlement or cracking due to the influence of hydrogen. A downstream thermal galvanizing in a molten zinc ("hot dip") leads in the usual workpieces made of water-resistant steels adversely to a high strength drop, which almost completely eliminates the tempering effect of hot forming The use of feueralumlnierter sheets has adversely the much poorer corrosion protection of aluminum compared to zinc, Difficulties in processing, for example, by the formation of adhesions on the forming tools, as well as high costs.

In contrast, the workpieces according to the invention made from the steel alloy according to the invention advantageously permit thermal galvanization without substantial loss of strength. It could even be shown that after hot-dip galvanizing in a zinc-aluminum melt at a process temperature of 400-450 ° C, the yield strength of the thermoformed components increased from 1,000 MPa to over 1,100 MPa. This can be produced at the same time advantageously high-strength and highly corrosion-resistant workpieces.

, Workpieces according to the invention also show the advantage of low-heat-treatment. By air hardening, the surface can be better protected against scaling, which has an advantageous effect on subsequent processing and coating processes.

Workpieces produced according to the invention are preferably suitable for seamless or welded steel pipes. In particular, workpieces can be used as thick-walled, torque-transmitting components in the form of drive shafts, gearbox parts or camshafts for motor vehicles.

Due to the good welding properties and low FestigkeRsabfall in the weld zone workpieces according to the invention for the construction of welded or brazed assemblies, in particular for the production of cohesively bonded frame structures for motor vehicles, such. Grid frame, where high strength requirements also exist in the region of the joining zone, are used. Likewise, the workpieces can be used advantageously as Achsbauteilen for motor vehicles.

Workpieces according to the invention may preferably be used in a forming tool for use as motor vehicle construction parts be hardened, since it is precisely in this area that essential process steps can be optimized.

The automotive components in the cured state may have tensile strengths Rm in a range of from 1200 MPa to 1500 MPa at a yield point Rp 0.2> 900 MPa and at an elongation at break A5 within a range of 8% -16%. In the case of structural parts in lightweight steel construction, tensile strengths Rm of> 1,000 MPa with a yield strength Rp 0.2> 750 MPa and with an elongation at break A5> 14% result in the air-hard state.

Claims (11)

1. Workpiece made from a high-strength steel alloy which consists of the following in proportions by weight carbon (C) 0.11 - 0.18 silicon (Si) 0.10 - 0.30 manganese (Mn) 1.60 - 2.20 phosphorus (P) < 0.0015 sulphur (S) < 0.010 chromium (Cr) 1.00 - 2.00 nitrogen (N) < 0.020 niobium (Nb) 0.020 - 0.060 boron (B) 0.001 - 0.004 titanium (Ti) 0.001 - 0.050 and iron and impurities caused by melting as the balance, wherein a forming production step takes place starting from a semi-finished product in the form of a sheet or tube with a ductile ferritic-perlitic structure, and wherein subsequently a heat treatment takes place by austenizing above Ac3, followed by a cooling in air, followed by an optional tempering treatment, wherein the workpiece produced in this way has a tensile strength Rm > 1200 MPa with an elongation at break A5 > 12%.
2. Workpiece made from a high-strength steel alloy having the chemical composition according to claim 1, wherein a forming production step takes place starting from a semi-finished product in the form of a sheet or tube with a ductile ferritic-perlitic structure with an elongation at break A5 > 25% at a forming temperature below 400°C, and wherein subsequently a heat treatment takes place by austenizing above Ac3, followed by a cooling in air at cooling rates between 0.5 and 5°C/sec, followed by an optional tempering treatment, wherein the workpiece produced in this way has a tensile strength Rm > 1200 MPa with an elongation at break A5 > 12%.
3. Workpiece according to claim 1 made from a high-strength steel alloy which consists of the following in proportions by weight carbon (C) 0.13 - 0.17 silicon (Si) 0.10 - 0.30 manganese (Mn) 1.80 - 2.10 phosphorus (P) < 0.0015 sulphur (S) < 0.010 chromium (Cr) 1.25 - 1.60 nitrogen (N) < 0.020 niobium (Nb) 0.020 - 0.040 boron (B) 0.001 - 0.004 titanium (Ti) 0.002 - 0.050 and iron and impurities caused by melting as the balance, wherein the forming production step takes place starting from a semi-finished product with an elongation at break A5 > 25% at a forming temperature below 400°C, wherein the cooling in air takes place at cooling rates between 0.5 and 5°C/sec.
4. Workpiece according to one of claims 1 to 3, characterised in that seamfree or welded tubes are used as the semi-finished product.
5. Workpiece made from a steel alloy according to one of claims 1 to 3, characterised in that, after heating to a temperature above Ac3, a hot forming and hardening takes place in a cooled forming mould at cooling rates > 10°C/sec, in order to produce a motor vehicle component.
6. Workpiece according to claim 5, characterised in that the motor vehicle component in the hardened state has a tensile strength Rm in a range from 1200 MPa to 1500 MPa, a yield strength Rp0.2 of more than 900 MPa and an elongation at break A5 in a range from 8% to 16%.
7. Workpiece according to claim 1, 2, 3, 5 or 6, characterised in that it is coated with a zinc-based coating by a melt-dip zinc coating step after forming, wherein the melt used for the melt-dip zinc coating contains a proportion of zinc of more than 50%, in that a melt bath temperature of more than 380°C is set, and in that the zinc-coated workpiece has a tensile strength Rm in a range from 1000 MPa to 1400 MPa, a yield strength Rp0.2 of more than 900 MPa and an elongation at break A5 in a range from 8% to 16%.
8. Use of the workpiece according to claim 4 for torque-transmitting components in the form of drive shafts, gear shafts or cam shafts for motor vehicles.
9. Use of the workpiece according to claim 4 for materially bonded framework structures for motor vehicles.
10. Use of the workpiece according to claim 4 as an axle component of motor vehicles.
11. Use of the workpiece produced according to one of claims 1 to 3 in light-weight steel construction, wherein the workpiece has a yield strength Rp0.2 > 750 MPa and an elongation at break A5 > 14%.